Supercapacitors, like batteries and conventional capacitors, are types of energy storage devices. The performance characteristics of an energy storage device (ESD) can be evaluated in terms of its energy density, the amount of energy that can be stored per unit weight or volume, and in terms of its power density, the rate at which an amount of energy can be transferred in or out of that unit weight or volume. Energy storage devices are commonly used as independent power sources or supplemental power sources for a broad spectrum of portable electronic equipment and electric vehicles.
Batteries are common energy storage devices for providing portable power. Energy storage in batteries is generally Faradaic, meaning that a chemical or oxidation state change occurs in the electroactive material. Although batteries have the potential for high energy density and can provide power over a wide range of voltages, their power density and the number of charging cycles are on-going limitations.
Capacitors are also common energy storage devices. Energy storage in conventional capacitors is generally non-Faradaic, meaning that no electron transfer takes place across an electrode interface, and the storage of electric charge and energy is electrostatic. Although capacitors have much higher energy transfer rates than batteries and can withstand orders of magnitude more charging cycles, they are limited by their low energy storage capacity, which is commonly on the order of microfarads or picofarads.
Supercapacitors, also known as ultracapacitors, electrochemical capacitors or electrical double-layer capacitors, are energy storage devices which combine the high energy storage potential of batteries with the high energy transfer rate and high recharging capabilities of capacitors. Supercapacitors can have hundreds of times more energy density than conventional capacitors and thousands of times higher power density than batteries.
Due to their high capacitance and high power, supercapacitors can be effective energy storage devices for a wide variety of applications. In low-voltage configurations of 5.5 volts or less, supercapacitors have applications in consumer electronics, such as backup power supplies for memories, microcomputers and clocks. In higher voltage configurations, supercapacitors have opportunities in electrical power load leveling, battery augmentation and pulse discharge applications, such as in wireless communication products. Other battery augmentation applications are possible in electric and fuel cell vehicles in which supercapacitors could be used to boost acceleration and regulate braking energy. Since supercapacitors can be recharged many times faster than rechargeable batteries and through many thousands of cycles, supercapacitors have applications in rechargers for such products as power tools, cordless phones, flashlights, electric shavers and other rechargeable devices. Supercapacitors are also expected to be useful in a wide range of robotic applications.
Energy storage in supercapacitors can be either Faradaic or non-Faradaic. Examples of supercapacitors that are of the Faradaic type include redox supercapacitors based on mixed metal oxides, such as ruthenium dioxide and other transition metal oxides. Redox supercapacitors can have both high energy density and power density. For example, an energy density of 8.3 Wh/kg and a power density of 30 kW/kg were achieved in a prototype 25-V capacitor built using RuO2.xH2O as the electrode material. (see B. E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publishers, NY, 1999, p. 266). However, due to the high cost, scarcity and toxicity of suitable metal oxides, supercapacitors based on carbon electrode materials may be preferred in many applications, especially those for power-related systems requiring higher capacitance capabilities.
In non-Faradaic supercapacitors, also known as electrical double-layer capacitors (EDLC), no electron transfer takes place across the electrode interface, and the storage of the electric charge and energy is electrostatic. In these supercapacitors, positive and negative charges accumulate electrostatically on the electrodes at the electrode-electrolyte interface. Electrical energy is stored in the electric double layer from charge separation, i.e. the electrostatic force between an ionically conducting electrolyte and a conducting electrode. The ions displaced in forming the double layers are transferred between electrodes by diffusion through the electrolyte.
In both the Faradaic and non-Faradaic supercapacitor systems, capacitance is highly dependent on the characteristics of the electrode material. Ideally, the electrode material should be electrically conducting and have a porous structure. The characteristics of the porous structure, including pore size, pore size distribution and pore volume fraction, can enable the formation of a large amount of surface area that can be used either for the development of the electrical double layer for static charge storage to provide non-Faradaic capacitance or for the reversible chemical redox reaction sites to provide Faradaic capacitance. Active electrode materials for supercapacitors include such materials as metal oxides, conducting polymers and various forms of carbon.
Electrochemical double-layer capacitors having electrode material based on high surface-area carbon, such as activated carbon powders and fibers, have shown promise. As electrode materials, carbon powders are generally more cost effective, but fibers and fabrics generally can produce higher performance supercapacitors.
The desirable attributes of the carbon electrode material include such factors as high surface area for the accumulation of charge at the electrode/electrolyte interface, good intra- and interparticle conductivity in the porous matrices, good electrolyte accessibility to the intrapore surface area, chemical stability and high electrical conductivity. The properties and performance of various carbonaceous materials as supercapacitor electrodes can vary widely depending on the carbon source, purity and treating conditions. For example, some possible carbonaceous materials suitable for electrode material include such materials as activated carbon, carbon black, carbon fiber cloth, highly oriented pyrolytic graphite, graphite powder, graphite cloth, glassy carbon and carbon aerogel.
One of the problems encountered with the use of different forms of activated carbon in electrodes is the lack of self-adhesion. To prepare an electrode with activated carbon, a polymeric binder is commonly incorporated in the electrode material. The use of insulating polymeric binders aggravates the power performance of the resulting supercapacitor by increasing the resistance of the electrode. In addition, the use of binder materials can fill or block the pores of the activated carbonaceous material and thereby reducing the available surface area available for double layer formation.
Generally, supercapacitors have energy densities in the range of about 1 to about 10 Wh/kg, which is about one-tenth of that of secondary cell batteries, which have energy densities of about 20 to about 100 Wh/kg. In contrast, supercapacitors generally have power densities in the range of about 1000 to about 2000 W/kg, which is about ten times higher than those of secondary cell batteries, which have power densities in the range of about 50 to about 200 W/kg.
Although the power density of supercapacitors is about ten times that of the secondary batteries, the energy density is smaller than that of secondary cells, and a serious deficiency for practical applications. Thus, there is a need for supercapacitors with high energy densities as well as high power densities.